This invention relates to rigid aromatic polyamide (aramid) foams and a process for their preparation.
Aromatic polyamides are known to have many desirable properties such as good resistance to oxidation at high temperature, a high melting point, low flammability, high stiffness and low sensitivity to solvents. Foam materials are known to be useful as thermal insulators and aramid foams suggest themselves for this use but the high melting points and low solubility of the aromatic polyamides makes fabrication of such foams difficult.
U.S. Pat. No. 3,770,669 discloses one approach to the preparation of polyamide foams. According to this patent polyamides are prepared from N,N'-dialkyl diamines and dicarboxylic acids or dicarbonyl chlorides. The resulting N,N'-dialkyl polyamides are heated at temperatures up to 380.degree. C., optionally in the presence of a salt of a strong acid and a weak base, e.g., pyridinium hydrochloride, to give a foam product. An example of an aramid foam is included. However, it has been found that this product is brittle and excessively flammable, apparently because of an insufficiently high molecular weight and incomplete dealkylation, respectively.
Foam materials combining the features of good mechanical strength and resistance to degradation at high temperatures in the presence of air have long been sought after. Such foams are particularly needed in uses where high temperatures are encountered in structures wherein mechanical strength of the foam is required. One such use is in solar collectors for the utilization of solar energy.